In general, a ballpoint pen tip as a writing portion is press-fitted and attached to one end (front end) of an ink containing tube in a refill for a writing tool to be used for a ballpoint pen or the like.
Further, the ink containing tube is made of a resin material such as polypropylene conventionally in order to achieve ease of molding and visibility of ink amount.
FIG. 14 illustrates an example of a conventional ballpoint pen refill in a cross-sectional view, and the example illustrated in FIG. 14 illustrates a configuration in which a ballpoint pen tip 2 is attached to a front end of an ink containing tube 1 with a joint member 3. Further, an outer peripheral face of the ink containing tube 1 is formed in a cylindrical shape to be flush. An inner diameter of the front end of the ink containing tube 1 to which the joint member 3 is fitted and an inner diameter of an axially central portion of the ink containing tube 1 forming the ink containing tube have substantially the same dimension to form a straight shape.
For example, JP A 2002-59682 (Patent Literature 1) discloses a ballpoint pen refill using such an ink containing tube having a straight inner circumferential surface.
A ballpoint pen refill is proposed in JP A 2006-175745 (Patent Literature 2), for example; the refill is configured such that an outer peripheral surface of an ink containing tube is similarly formed flush in a cylindrical shape and an inner diameter of a front end of the ink containing tube to which a ballpoint pen tip is fitted is bored to be larger than an inner diameter of a central portion of an axial direction forming an ink containing portion.
In the above-described ballpoint pen refills, the front end in particular of the ink containing tube to which the ballpoint pen tip is fitted needs to have a certain thickness to ensure its strength. Consequently, there arises a practical problem that it is difficult to ensure a sufficient amount of ink stored because the front end has the fixed thickness in case of the ballpoint pen refills having the configuration disclosed in Patent Literatures 1 and 2.
A ballpoint pen refill disclosed in the Patent Literature 3 exemplifies a refill for a writing tool that takes into consideration such that a sufficient amount of ink can be stored. According to the ballpoint pen refill described in Patent Literature 3, however, it is necessary to individually prepare an ink containing tube having a size corresponding to the barrel for each model, since an ink containing tube having a fan-shaped cross section orthogonal to the axis is used, which is problematic in terms of versatility.
In addition, it is difficult to impart flexibility to the ink containing tube having a fan-shaped cross section orthogonal to the axis. As a result, when the ink containing tube is used for a composite writing tool that houses plural refills in a barrel, there arises a technical problem where a smooth operation can hardly be expected; that is, it is difficult to operate the ballpoint pen tip to alternately protrude and retract through a distal end opening of the barrel.